题目

区别空气、氧气和二氧化碳这三瓶无色气体最简单的方法是（    ） A.测三瓶气体的密度            B.用燃着的木条分别放入瓶中 C.分别倒入澄清石灰水          D.用带火星的木条分别放入瓶中 答案：BRewrite the sentences as required. (按要求改写句子，每空格一词)： You needn't do every one of these exercises. (意思不变) It is not ________ ________ you to do every one of these exercises.